Employee went to trouble shoot a leveling sensor located at the top of a silo. He claimed up the ladder and open the hatch at the top of the silo. When he looked inside a gust of wind caught a hold of the hatch door (30"" x 30"" x 1/4"" steel plate on hinges) striking him in the back of the head causing him to hit the bridge of his nose on the steel frame opening of the hatch.